Flame retardant silicone compositions

Abstract: To a silicone composition comprising (A) an organopolysiloxane having at least two alkenyl groups in a molecule, (B) an organohydrogenpolysiloxane having at least two hydrogen atoms each directly attached to a silicon atom in a molecule, and (C) a platinum catalyst, (D) a compound having a phenyl group and a secondary or tertiary amino group directly attached thereto is added to render the composition flame retardant. The composition cures into a transparent product having improved flame retardancy meeting the UL standard and is suitable for use in the protection of electronic parts like ICs and hybrid ICs.

Description: This specification relates to flame retardant silicone compositions, their preparation and use. In particular it concerns such compositions which are curable to a transparent product, e.g. so as to be suitable for the protection of ICs or hybrid ICs. For addition curing type silicone rubber compositions which cure into elastomers, a number of techniques are known for rendering them flame retardant. The most basic technique is to add platinum compounds to organopolysiloxanes. The addition of benzotriazoles, hydrazines and similar compounds is known effective for improving flame retardancy. Fumed titanium oxide, cerium oxide, cerium hydroxide, iron oxide and carbon powder are also known effective. All the prior art silicone rubber compositions of this type contain reinforcing inorganic fillers such as fumed silica and quartz powder. Flame retardant silicone compositions essentially free from reinforcing inorganic fillers are unknown. EP-A-153700 describes organopolysiloxane compositions consisting of an organopolysiloxane containing at least two silicon-bonded alkenyl groups per molecule, an organohydrogenpolysiloxane containing at least two organohydrogensiloxane or hydrogensiloxane units per molecule, a platinum-type catalyst, an addition-reaction retarder which may include benzotriazole, and a photosensitizer. Flame retardancy is not discussed. One known transparent material based on organopolysiloxane is silicone gel which finds use as a filler for parts used in optical equipment, a protective material for hybrid Ics used in electric and electronic parts to be mounted on automobiles, and a protective sealing agent for commercial power modules. Although flame retardancy is essentially required in these fields, prior art silicone compositions could not meet the requirement. It was a common approach in the art to ensure that parts as a whole are flame retardant. As greater attention is now paid to the environmental problem, the requirement for flame retardancy becomes severe. There is a need for a silicone composition which offers not only transparency, but also flame retardancy. Working with silicone compositions comprising (A) an organopolysiloxane having at least two alkenyl groups in a molecule, (B) an organohydrogenpolysiloxane having at least two hydrogen atoms each directly attached to a silicon atom in a molecule, (C) a platinum catalyst, and curing to a transparent product, we have found firstly that when (D) a compound having a phenyl group and a secondary or tertiary amino group directly attached thereto is added to the composition, we can obtain flame-retardant compositions even without the use of reinforcing inorganic fillers, such as fumed silica and quartz powder, which are conventionally regarded as essential in all flame retardant silicone compositions. We have also found that when the non-functional low-molecular weight siloxanes having a degree of polymerization of 3 to 10 (D 3 to D 10 ) in components (A) and (B) is controlled low, the composition is improved in flame retardancy e.g. as demonstrated by a flame retardant level rated as UL-94 V-0. One specific aspect proposed herein provides a flame retardant silicone composition which cures to a transparent product, comprising (A) 100 parts by weight of an organopolysiloxane having at least two alkenyl groups in a molecule, (B) about 0.1 to about 30 parts by weight of an organohydrogenpolysiloxane having at least two hydrogen atoms each directly attached to a silicon atom in a molecule, (C) about 0.1 to about 1,000 parts by weight per million parts by weight of the composition of a platinum catalyst, and (D) about 0.01 to about 1 part by weight of a compound having a phenyl group and a secondary or tertiary amino group directly attached thereto. Components (A) and (B) contain non-functional low-molecular weight siloxanes D 3 to D 10 in a total amount of less than about 1,000 parts by weight per million parts by weight of components (A) and (B). EXPLANATIONS; PREFERRED AND OPTIONAL FEATURES Component (A) is an organopolysiloxane having at least two alkenyl groups in a molecule. Preferably component (A) is an organopolysiloxane of the following formula. R a R 1 / bSiO (4-a-b)/2 In the formula, groups R are alkenyl, preferably lower (e.g. C 1-5 ) such as vinyl, allyl and butenyl groups, with the vinyl group being preferred. R 1 is generally methyl or phenyl group. We find that unless groups R 1 are substantially all methyl or phenyl, satisfactory flame retardancy is not reliably achieved. Trifluoropropyl, for example, is not effective. Letters a and b are positive numbers which are selected such that component (A) contains at least two R's (alkenyl groups). More preferably, 1.9 < a < 2.4 and 0.001 < b < 0.05. The sum of a + b is in the range of 1.9 to 2.4. The organopolysiloxane may be either a linear one or a branched one containing a RSiO 3/2 or SiO 4/2 unit. Preferably the organopolysiloxane has a viscosity of about 100 to about 100,000 centistokes at 25°C, especially about 100 to about 10,000 centistokes at 25°C. With a viscosity of less than 100 centistokes, the resulting cured product would be brittle. With a viscosity of more than 100,000 centistokes, the composition would be less flowing, difficult to cast and less flame retardant. It is acceptable that in addition to component (A), an organopolysiloxane containing only one alkenyl group in a molecule or an organopolysiloxane free from an alkenyl group is added for hardness adjustment. Component (B) is an organohydrogenpolysiloxane having at least two hydrogen atoms each directly attached to a silicon atom in a molecule, which serves as a crosslinking agent by reacting with component (A). No particular limit is imposed on the molecular structure and any of conventional well-known organohydrogenpolysiloxanes including linear, cyclic and branched structures may be used. Preferably the organohydrogenpolysiloxane has a methyl and/or phenyl group as a substituent attached to a silicon atom other than hydrogen. Component (B) is preferably added in amounts of about 0.5 to about 5.0 equivalents, especially about 0.8 to about 2.0 equivalents per alkenyl group in the entire composition. Less than 0.5 equivalents of component (B) on this basis may provide a lower crosslinking density and adversely affect the heat resistance of cured product. More than 5.0 equivalents of component (B) may give rise to a bubbling problem by inducing dehydrogenation reaction and also adversely affect the heat resistance of cured product. Quantitatively stated, about 0.1 to about 30 parts by weight of component (B) is typically added per 100 parts of component (A). It is preferred that the sum of hydrogen atoms each directly attached to a silicon atom in component (B) and alkenyl groups in component (A) is at least 5 because if the sum of hydrogen atoms and alkenyl groups is less than 5, sufficient crosslinking to form a three-dimensional structure would not take place, resulting in an unsatisfactory cured product. We have found that the content of non-functional, low-molecular weight siloxanes in components (A) and (B) is a significant factor dictating flame retardancy. In general, component (A) or organopolysiloxane is prepared by effecting equilibration of cyclopolysiloxanes in the presence of a strongly basic catalyst (e.g., potassium hydroxide, tetraalkylammonium hydroxides, and tetraalkylphosphonium hydroxides) or a siliconate compound thereof, followed by neutralization and deactivation of the catalyst. From the resulting polymer, concomitant low-molecular weight siloxane by-products are generally removed by stripping at elevated temperature in vacuum. While the level of removal depends on economy, currently available polymers of commercial grade contain low-molecular weight compounds having 3 to 10 silicon atoms in a molecule in a total amount of several thousand to several ten thousand ppm. We have found that significant flame retardancy improvement is achieved by controlling the content of low-molecular weight compounds to about 1,000 ppm or lower, especially about 500 ppm or lower. The term ppm is parts by weight per million parts by weight of the entire polymer. We thus recommend that the total content of non-functional low-molecular weight siloxanes having a degree of polymerization of 3 to 10 (D 3 to D 10 ) in components (A) and (B) is controlled to less than about 1,000 ppm, especially less than about 500 ppm. For removing the majority of low-molecular weight siloxanes to provide such a limited content, enhanced separation techniques including critical extraction, low-molecular weight component extraction with the aid of solvent, and stripping by thin-film evaporation are recommended rather than conventional commonly employed evaporation techniques. After low-molecular weight compounds have been removed by vacuum heat treatment such as thin-film evaporation, the amount of silanol remaining in component (A) is naturally reduced. The low-molecular weight compound content can be measured by FID gas chromatography. Component (C) is a platinum catalyst for promoting addition reaction or hydrosilation between components (A) and (B). The catalyst may be as known in the art and selected from platinum and platinum compounds, for example, platinum black, chloroplatinic acid, alcohol modified chloroplatinic acid, and complexes of chloroplatinic acid with olefin aldehydes, vinylsiloxanes and acetylene alcohols. The catalyst is used in a catalytic amount which may be suitably selected in accordance with a desired curing rate. In general, the platinum catalyst is added in an amount of about 0.1 to about 1,000 ppm, preferably about 10 to about 200 ppm of platinum, based on the weight of the entire composition. To achieve flame retardancy, the composition further contains (D) a compound having a phenyl group and a secondary or tertiary amino group directly attached to the phenyl group. Although the interaction of this compound is not well understood, it was found by an actual burning test to be quite effective for rendering the composition flame retardant. While such nitrogenous compounds as hydrazines, azobisisobutyronitrile, benzotriazoles, and aminosiloxane oil are known as a component effective for imparting flame retardancy to conventional thermosetting silicone rubber compositions using peroxides, not all of these are effective in addition-curing compositions. The nitrogenous compounds which are effective in the present composition are limited to those compounds having a secondary or tertiary amino group adjacent to a phenyl skeleton. The secondary or tertiary nitrogen may be part of a heterocyclic ring, e.g. in an azole such as benzotriazole or benzimidazole, or in phenoxazine or 2-phenylindole. Or, it may be non-cyclic e.g. in an N-substituted aniline or anilino compound such as N-allylaniline, p-anilinophenol, m-anilinophenol or 2-anilinoethanol. Siloxane-modified versions of these may also be used. Component (D) is used in an amount of about 0.01 to about 1 part by weight, preferably about 0.05 to about 0.5 parts by weight per 100 parts by weight of component (A). In the present compositions, reinforcing fillers which are essential in conventional silicone compositions, e.g. silicas such as fumed silica and finely divided silica by the wet system, are preferably omitted because surprisingly they adversely affect flame retardancy. Insofar as transparency and other desirable properties are maintained, such additives as carbon black, red iron oxide, cerium oxide, cerium hydroxide, titanium oxide, and titanates may be added. Addition of alumina and analogues dose not necessarily raise a problem insofar as the desired properties are not impaired, but heavy loading of alumina would offset the advantage associated with the removal of low-molecular weight siloxanes. It is then desired that these additives be added in limited amounts of 0 to about 5 parts, especially 0 to about 0.5 parts by weight per 100 parts by weight of component (A). The composition can be cured at room temperature or at an appropriate temperature corresponding to its application, yielding a transparent, flame retardant silicone rubber or gel. Therefore, the present teachings make it possible to impart flame retardancy to transparent silicone rubber which has been regarded difficult to render flame retardant in the prior art. Since flame retardancy can be imparted to silicone gel which has been widely used for the protection of integrated circuits, these electronic parts are further improved in reliability. EXAMPLE Examples illustrating the present concepts are given below. Examples 1-18 & Comparative Examples 1 - 4 The following components were furnished and silicone compositions of the formulation shown in Tables 1 to 3 were prepared therefrom. The compositions were cured by heating at 150°C for 60 minutes and the cured products were evaluated for hardness, penetration, and flame retardancy by the methods described below. The results are shown in Tables 1 to 3. Components (Note: the D 3 -D 10 content is a content of low-molecular weight siloxanes D 3 to D 10 .) Component (A) A-1 Dimethylpolysiloxane end-blocked with a dimethylvinylsilyl group having a viscosity of 100 centistokes at 25°C and a D 3 -D 10 content of 840 ppm. A-2 Dimethylpolysiloxane end-blocked with a dimethylvinylsilyl group having a viscosity of 1,000 centistokes at 25°C and a D 3 -D 10 content of 50 ppm. A-3 Dimethylpolysiloxane end-blocked with a dimethylvinylsilyl group having a viscosity of 1,000 centistokes at 25°C and a D 3 -D 10 content of 580 ppm. A-4 Dimethylpolysiloxane end-blocked with a dimethylvinylsilyl group having a viscosity of 1,000 centistokes at 25°C and a D 3 -D 10 content of 750 ppm. A-5 Dimethylpolysiloxane end-blocked with a dimethylvinylsilyl group having a viscosity of 1,000 centistokes at 25°C and a D 3 -D 10 content of 7,280 ppm. A-6 Dimethylpolysiloxane end-blocked with a dimethylvinylsilyl group having a viscosity of 10,000 centistokes at 25°C and a D 3 -D 10 content of 790 ppm. A-7 Organopolysiloxane end-blocked with a dimethylvinylsilyl group, containing 5 mol% of a diphenylsiloxy group and the remainder of a dimethylisiloxy group, having a viscosity of 3,000 centistokes at 25°C and a D 3 -D 10 content of 830 ppm. A-8 Dimethylpolysiloxane blocked with a dimethylvinylsilyl group at one end on average having a viscosity of 800 centistokes at 25°C and a D 3 -D 10 content of 620 ppm. Component (B) Methylhydrogenpolysiloxane of the following average structural formula having a viscosity of 120 centistokes at 25°C and a D 3 -D 10 content of 2,560 ppm. Component (C) A dimethylpolysiloxane solution of a chloroplatinic acid-vinylsiloxane complex containing 1% by weight of platinum. Component (D) D-1: benzotriazole D - 2: benzimidazole D-3: siloxane-modified p-anilinophenol D - 4: aniline R-812: trimethylsilylated hydrophobic fumed silica commercially available from Degussa A.G. CB: acetylene black HS-100 commercially available from Denki Kagaku K.K. Hardness JIS A scale hardness Penetration penetration of a 1/4 cone according to JIS K-2220 Flame retardancy In the flame retardant test, bars of 125 mm × 13 mm cut from a cured sheet of 3 mm thick were subjected to the vertical burning test procedure set forth in Underwriters' Laboratories, Inc. Bulletin 94, Burning Test for Classifying Materials (UL-94). The time of flaming and/or glowing after removal of the igniting flame was measured. The time reported in Tables 1 to 3 is an average of five bars. All the cured products were substantially transparent.

Claims: A flame retardant silicone composition comprising (A) 100 parts by weight of organopolysiloxane having at least two alkenyl groups per molecule, (B) 0.1 to 30 parts by weight of organohydrogenpolysiloxane having per molecule at least two hydrogen atoms each directly attached to silicon atom, (C) 0.1 to 1,000 parts per million by weight of the composition of platinum catalyst, and (D) 0.01 to 1 part by weight of a compound having a phenyl group and a secondary or tertiary amino group directly attached thereto, said components (A) and (B) containing non-functional low-molecular weight siloxanes having a degree of polymerisation of 3 to 10 in a total amount of less than 1,000 parts per million by weight of components (A) and (B), and the composition being curable to a transparent product. A flame retardant silicon composition according to claim 1 in which components (A) and (B) contain said non-functional low-molecular weight siloxanes in a total amount of less than 500 parts by weight per million parts by weight of components (A) and (B). A flame retardant silicone composition according to claim 1 or 2 in which component (A) is an organopolysiloxane having the compositional formula, R a R 1 b SiO (4-a-b)/2 in which groups R are alkenyl groups having up to 5 carbon atoms and groups R 1 are selected from methyl and phenyl groups, a and b being positive numbers selected such that component (A) has at least two alkenyl groups R per molecule the sum of a+b is between 1.9 and 2.4, 1.9 < a < 2.4 and 0.0001 < b < 0.05. A flame retardant silicone composition according to any one of claims 1 to 3 in which (A) has a viscosity of 100 to 100,000 centistokes at 25°C. A flame retardant silicone composition according to any one of claims 1 to 4 comprising a second organopolysiloxane, the second organopolysiloxane either containing one alkenyl group in a molecule or being free of alkenyl groups. A flame retardant silicone composition according to any one of claims 1 to 5 in which in component (B) substituents on silicon are selected from methyl and phenyl groups. A flame retardant silicone composition according to any one of claims 1 to 6 in which component (D) is benzotriazole, benzimidazole, phenoxazine, 2-phenylindole, N-allylaniline, p-anilinophenol, m-anilinophenol, 2-anilinoethanol or siloxane-modified versions of any one thereof. A flame retardant silicone composition according to any one of Claims 1-7 in which an additive selected from carbon black, red iron oxide, cerium oxide, cerium hydroxide, titanium oxide, titanates and alumina is present, the additive being added in amounts up to 5 parts per 100 parts by weight of component (A). A method of preparing a flame retardant silicone composition according to any one of claims 1 to 9 in which non-functional low-molecular weight siloxanes as defined in claim 1 are removed by critical extraction, low-molecular weight component extraction with the aid of a solvent, or thin-film evaporation.

Inventors: FUJIKI HIRONAO, HARA HIROYASU, IKENO MASAYUKI, SATOH KAZUYASU, FUJIKI, HIRONAO, HARA, HIROYASU, IKENO, MASAYUKI, SATOH, KAZUYASU
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1994
Main classification: C08K5
IPC codes: H01L23, C08K5, C08G77, C08L83